In hot rolling mills, systems are typically provided for controlling operating variables (e.g., rolling force) in the finishing mill stage in order to produce a strip having a uniform predetermined gauge. The most common of these systems have very poor response times and are useful only to correct gauge variations having wavelengths of the same order of magnitude as the length of the strip. For the correction of gauge errors which having shorter wavelengths, very complex and expensive systems are used. Because these systems involve changes in rolling forces, they typically have poor response times and do not satisfactorily correct gauge variations of short wavelengths.
In a conventional rolling mill, looper rolls are positioned between adjacent mill stands in the finishing mill stage. These rolls are conventional devices which maintain tension in the section of the strip between the adjacent mill stands in order to hold the strips along the center line of the mill. The looper rolls are set to apply the greatest amount of tension without causing the strip to yield.
The looper rolls also ensure that the mass flow out of a mill stand equals the mass flow into the adjacent downstream mill stand. When more mass is flowing out of the upstream mill stand than is flowing into the downstream stand, the strip will become slack and the looper roll will change position in an attempt to maintain tension In response to this change of position, the speed of one or both of the adjacent mill stands is changed in order to equalize the mass flow and return the looper roll to its original position. A similar adjustment is made when tension becomes too high from unequal mass flow.
In a recent attempt to reduce the well-known effect of width "neckdown" when strips are threaded into the finishing mill stage, the tension on the strips from looper rolls has been increased beyond the strip's yield point. Immediately after a strip is threaded or between stands, the associated looper roll applies only normal tension in order to minimize the "neckdown" effect. Thereafter, the tension is raised in order to cause the strip to yield so that both gauge and width are reduced. The results are strips characterized by more uniform gauge and width than previously possible. In this system, the high tension is set at a predetermined level which remains constant.
Although the foregoing system of high tension rolling provides more uniform gauge and width dimensions and thereby provides an inexpensive system to reduce the magnitude of relatively short wavelength variations of gauge, it has no ability to adjust to dynamic changes in the system which cause gradual changes in gauge and width when the strip is considered as a whole. Sometimes the drifting of the gauge and width dimensions results in substantial differences when various areas of a strip are compared. These differences can result in strips being scraped because areas of it are beyond the customer's specified tolerances.